The present invention relates to Global Navigation Satellite System (GNSS) receivers, and more particularly, to methods and apparatus for obtaining accurate GNSS time in a GNSS receiver.
One of the most important issues related to GNSS receivers is how to obtain accurate GNSS time when a GNSS receiver enters a start up mode from a power-off mode. Typically, within the GNSS receiver, all components except a real time clock (RTC) are powered down in the power-off mode. According to the related art, a common way to get an initial GNSS time when the GNSS receiver is powered on is by reading the RTC time provided by the RTC as the Coordinated Universal Time, which is referred to as the UTC time, and by further converting the UTC time derived from the RTC time into a rough initial value of the GNSS time directly. Thus, some problems are introduced when implementing according to the related art. For example, UTC leap seconds are unknown. In addition, the cycle of the RTC is typically around several microseconds and the resolution of the RTC is typically around several tens to several hundreds parts per million (PPM), causing the aforementioned initial value of the GNSS time to be unacceptable. Additionally, during time synchronization between the RTC time and real GNSS time, a time delay typically exists, causing the aforementioned initial value of the GNSS time to be inaccurate.